Circulation of two genotypes of porcine reproductive and respiratory syndrome virus (PRRSV) within swine herds in Poland

Porcine reproductive and respiratory syndrome (PRRS), now recognized as a significant disease affecting swine worldwide, is caused by the PRRS virus. One of the most important ways of controlling the disease is through the use of modified live vaccines based on European (PRRSV-EU) or American (PRRSV-US) genotypes. There is increasing data suggesting the limited efficacy and safety of some of these bio-products. The viral strain present in the vaccine may be transmitted from vaccinated to healthy animals. Prolonged in-vivo passaging can cause the virus to revert to a less attenuated form which can spread both within a particular swine herd as well as being transmitted between herds. The article reports the first evidence of PRRSV-US in Poland and suggests that this viral strain originated from a modified live vaccine. It has been demonstrated that two PRRSV genotypes can circulate within a single swine herd indicating limited cross-protection between these two types. A new single tube reverse transcription multiplex nested PCR (RT-M-nPCR) method was applied for genotype discrimination which allowed sensitive detection and genotyping.